Apparatus and method for correcting imperfection in a polygon used for laser scanning

ABSTRACT

An electro-optical light scanning system using a modulated laser illumination source directed upon a multifaceted rotating polygonal mirror or polygon. The mirrored facets reflect the impinging light toward a moving photoreceptor and forms a raster of scan lines as the photoreceptor moves. The system incorporates sensing optics and closed loop electronics for correcting inaccuracies in the position of the reflected light resulting from defects in the angular relationship between the plane of the facets and that of the rotating axis of the polygon as well as those errors due to inherent angular misalignment between each of the facets of the rotating polygon. The invention also encompasses amplitude modulation for varying the intensity of the laser illumination in conjunction with the acousto-optical modulation for maintaining a constant level illumination and/or for varying the spot size.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a laser spot scanning system for communicatinginformation to a scanned medium and especially to a scanning systemwhich utilizes reflected light from a multifaceted rotating polygon.

In particular, the invention is directed to a spot scanning system usingacousto-optical and electro-optical means and methods to compensate fordefects in facet to facet relationship and facet to axis error.

2. Description of the Prior Art

A recurring problem in scanner systems is to reduce or eliminate errorintroduced as a result of inherent defects in the construction ofrotating polygonal mirrors. Such defects occur usually in the angularrelationship between adjacent facets (facet-to-facet) and between facetplanes and the polygonal rotational axis (facet-to-axis). A typicalsolution is to employ nonspherical optics to partially correct theeffects of facet angular error as is shown in the apparatus described inthe U.S. Pat. No. 4,002,830. Another situation is the use of opticalreflecting or refracting elements pivotally mounted in the path ofradiation utilizing electromechanical devices that are energized bytimed electrical signals such that the refracting element is pivoted tocorrect the scanning errors caused by angular defects in the rotatingmirror. The control of electromechanical devices is preprogrammed tomake proper adjustments and the fabrication of these optical systems isthus expensive, as is alignment of the same.

In contrast to systems which use an encoder and logic to correct forerror and operate in an open-loop manner by applying a predeterminedcorrection factor, the instant system employs sensing optics and afeedback loop to correct the position of the raster scan lines.

SUMMARY OF THE INVENTION

The invention relates to an electro-optical light scanning system usingmodulated laser illumination and is particularly adaptable for nonimpactand fascimile printing. The light source, such as a laser beam, isacousto-optically modulated in accordance with selected input data. Thelaser beam so modulated is directed toward a multi-faceted polygondriven at a constant angular velocity. As the successive mirrored facetsof the polygon are illuminated, the light reflected generates aplurality of scan lines formed by successive dots which move across amoving photoreceptor and which are modulated to thus generate charactersor a recorded likeness of an original image.

In order to reduce scan line displacement errors and further toestablish a geometric reference for synchronizing the writing logic withthe position of the laser beam, this invention utilizes sensing opticsand closed loop electronics for corrective compensation.

In particular, the facet-to-axis polygon error is detected by a spotposition sensor, e.g., a split detector positioned within the scanformat plane or optical equivalent and within the scan path. Before thereflected light traverses the photoreceptor, its longitudinal path axisis detected by a spot position sensor which includes a division parallelto the scan direction. A signal is generated which is then processed andused for applying corrective frequency modulation to the acousto-opticalmodulator to deflect the laser beam to the desired position prior toinitiation of the scan line upon the photoreceptor.

The facet-to-facet polygon error is minimized by an optical edge startdetector, such as a split photocell, positioned in the scan path andhaving a division perpendicular to the direction of the scan lines. Theedge detector senses the passing of each scan line and establishes ageometric reference position for the beginning of each scan which isindependent of any facet-to-facet error. The laser modulation for on-offgating and digital input from the buffer memories can thus besynchronized for writing on the photoreceptor.

An edge stop detector, similar to the start detector, is located at aterminal point beyond the photoreceptor. By measuring the time it takesfor the scan spot to travel from the start to the stop edge detectors,the time of the flight of the scan spot is thusly determined. Thisinformation may be electronically interpreted and applied through afeedback loop to control the drive motor circuitry of the polygon andfor maintaining constant velocity.

An advantage therefore of the present invention over the prior artsystems for polygon encoding is that the individual facets of thepolygon do not have to be characterized by preprogramming correctiveadjustments. All accuracy is referrenced to the edge detector; thereforethe frequency modulator is not required to have long term stability.Drift is removed from consideration as well as long term effects as longas they remain in the control range of the system.

BRIEF DESCRIPTION OF THE DRAWING

In an accompanying drawing in which is shown a preferred embodiment ofthe invention:

FIG. 1 is a diagramatic representation of the major components of anoptical scanning device according to the present invention, the angle ofreflection of the beam being distorted for purposes of illustration;

FIG. 2 is a schematic representation of the longitudinal travel of amodulated laser beam approaching and crossing a spot detector andphotoreceptor;

FIG. 3 is a schematic diagram of the spot correction logic; and

FIG. 4 is a diagramatic illustration of spot displacement duringcorrective modulation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, FIG. 1 illustrates an overall view of thescanning system of this invention. The light source, such as a laser 10,which may be a 3 mw helium-neon laser, generates a collimated beam 12 ofmonochromatic light which is directed through a neutral density filter14 to control the light intensity. The beam 12 then passes through amodulator 16, such as an acousto-optical modulator. The beam 12 is nextdirected through a first lens 20 and intercepted by a knife edge 22placed at the focal point of the first lens 20. The knife edge 22 isemployed for stopping the zero order Bragg beam. The first order beam isthus separated and passes the knife edge 22 unattenuated. An example ofa commercially available acousto-optic modulator is Model 1209 by IsometCorp., Springfield, Va., which provides a built-in Bragg angleadjustment. The modulator 16 can typically be operated by a digitaldriver, such as Model no. 220 available from Isomet Corp. whereintransistor-transistor logic compatible digital input controls an RFswitch for on-off gating of the modulator 16. Another acousto-opticalmodulator is Model 304 manufactured by Coherent Associates, Danbury,Conn.

It is desirable to use the first order beam to produce a spot becausethe position of the spot can be displaced in accordance with frequencymodulation applied to the modulator which will selectively deflect thebeam 12 in a desired direction such as indicated by the arrows a,b. Thefirst order beam 12 is then directed toward a second lens 24 whichdirects a converging beam onto a reflecting face or facet 26 of arotating polygonal mirror, herein reffered to as a polygon 28. Thepolygon 28 is continuously driven by a motor drive 30 and preferably ismaintained at a constant velocity. In the preferred embodiment as shown,the polygon 28 has thirty facets 26 and is designed for generatingapproximately 240 scan lines per second. A moderate spot velocity ispreferred for implementing the optical spot sensing and closed loopfeedback correction circuitry.

The beam 12 is thus reflected successively from each of the facets 26 ofthe rotating polygon 28 and onto a photoreceptor 32. The reflection ofthe beam 12 from the polygon 28 is distorted for purposes ofillustration as it will be appreciated that the incident beam andreflecting beam will be in the same plane rather than at an angle to oneanother as indicated by FIG. 1. The modulated beam 12 may appear as asuccession of dots 34 which will generate a scan line forming a rasteracross the moving photoreceptor 32. The photoreceptor 32 may be anyimage plane and can be mounted on a rotating drum such as for use withan electrophotographic copier.

It should thus be apparent that the light scanning system of the presentinvention can be readily interfaced with an electrophotographic copierhaving panchromatic photoreceptors and can thus function as a highquality nonimpact printer.

It is well known that various types of errors are inherent in thegeometric fidelity of a commercially available rotating polygon. Inparticular, deviation in parallelism of each facet relative to the axisof rotation introduces a facet-to-axis error and the resulting scanlines will correspondingly contain these inaccuracies which manifestthemselves as alignment deviations from a desired scan line travel axis,i.e., line to line spacing variation. The present invention provides aspot correction assembly 36 for optically detecting and correcting forthese facet-to-axis errors. The spot correction assembly 36 in thepreferred embodiment, is provided with an optical detector in the formof a split detector 38 optically positioned in the scan format plane anddivided in half to form two cells A,B with a common electrode. Adivision C formed between the two cells A,B is registered with thedesired scan path axis and has a dimension substantially less than thediameter of the spot 34. A signal will thus be generated from either orboth cells A,B when the spot 34 sweeps the split detector 38. Since thealignment of division C is parallel to the scan direction, the divisionC provides a reference for indicating deviations of spot 34 from thedesired travel axis on the photoreceptor 32.

Referring now to FIG. 2, maximum allowable uncorrected facet-to-axisangular error may cause the spot 34 to fall anywhere within a transversezone D, within the light sensitive area of split detector 38. Successivescans as determined by the rotating polygon 28 are a distance D which isgreater than the width D' of distance from the outside edge 33 of thephotoreceptor 32 to the outside edge 39 of the split detector 38 by atleast the greatest facet-to-facet deviation. It should thus be evidentthat the correction of each successive spot 34 is achieved during a"dead" time, i.e., the period of travel prior to traversing thephotoreceptor 32.

A typical logic circuit implementing the present invention for controlof an acousto-optical deflector to provide compensating deflection ofthe laser beam such that the spot 34 will exit the split detector 38 inregistration with the division C between the individual cells A and B isshown in FIG. 3. When an uncorrected spot 34 of the laser beam entersthe detector 38, a comparator 40 compares the signal generated at eitherphotocell A or B with a reference voltage V_(r) and provides a lowoutput signal to an inverter 42 to generate a high enabling signal at anAND gate 44. A system clock 45 provides correction count pulses as asecond input to the AND gate 44. The presence of the spot 34 at eithercell segment A or B thus provides a high signal at the AND gate 44enabling clock pulses to pass through the AND gate 44 and register at acounter 46. The instantaneous count of the counter 46 drives a digitalto analog converter 48 which in turn provides an analog correctionsignal via a voltage controlled oscillator 50, an amplitude control 51,and an amplifier 52 to an acousto-optical deflector 54 which isincorporated into the modulator 16. The beam 12 will thus be displacedin the appropriate direction a or b. The amplitude control 51 isconnected to a summer 53 that measures the amount of light falling on Aand B of the split detector 38 to maintain the light output constant tothe acousto-optical deflector 54.

It should be appreciated that the direction of count of the counter 46determines the direction of corrective deflection applied by thedeflector 54. In order to control the direction of count, a secondcomparator 56 compares the output of cell A with respect to the outputof cell B. If the terminal spot 34 of the laser beam enters cell A, theoutput of cell A will be greater than that of cell B, and the comparator56 will provide a low output. The low output of comparator 56 determinesthe count direction of the counter 46. As the deflecting correction isapplied, the spot 34 progresses towards cell B while translating acrossthe detector 38. As soon as the spot 34 crosses into cell B, the signalof cell B will be greater than the signal of cell A which causes thecomparator 56 to switch to a high output. The high output of thecomparator 56 reverses the direction of the counter 46 and thus providesan opposite direction of corrective deflection of the laser beam suchthat the laser beam will progress towards cell A. Thus, the spot 34 willtrack the division C until it exits from the detector 38 at which timethe clock 45 is disabled and the correction value for the particularfacet 26 is digitally stored in counter 46 until the next uncorrectedspot enters the detector.

The control process is further detailed in FIG. 4. The spot 34 is shownentering the split detector 38 at cell A. The displacement of the spot34 resulting from an incremental change in the counter is reflectedthrough the closed loop control circuitry through a deflection in eitherdirection a or b. The uncertainty of the exit point of spot 34 is ±D_(s)due to the digitization of the signal. The amount D_(s) is less than thetolerable error. With a given voltage controlled oscillator andacousto-optical modulator, D_(s) can be varied by changing the scalingfactor of the digital to analog converter. Since the most extreme errorwould be equivalent to D1/2, the number of correction steps to bring thespot 34 to the division C is a maximum of n=D1/2D_(s). Assuming it takesa given time (t_(s)) to perform a corrective step, the total time is(n)(t_(s))=D₁ t_(s) /2D_(s). If the spot velocity is V_(s), then theminimum length of photocell required is D₂ =D₁ t_(s) /2D_(s) (V_(s)). Atypical value for V_(s) is 2200 inches per second.

Other alternative closed loop means of control include a successiveapproximation technique which converges more rapidly than the abovedescribed counter system.

A further method capable of converging more rapidly is the use of aprecision sensing detector such as United Detector TechnologyPIN-SC/10D, which senses the centroid of a light spot and gives analogoutput proportional to spot position. These outputs can be digitizeddirectly and coupled to a voltage controlled oscillator withoutintervening conversions.

Scan line spot detection will now be discussed with reference to FIG. 1.Since the modulator 16 is controllable by internal logic whichdetermines exposure on the scan format 32, an edge detector 58 ispositioned adjacent the leading edge of an exposure slot 60 formed in anopaque shield 61 for indicating when the spot 34 is at the preciselocation. The edge detector 58 and a logic circuit can thus be used asan implement to synchronize the internal logic with the location of thescan line. It should therefore be obvious that each sweep of the scanline is independently referenced and will thus negate any facet-to-facetpolygon error which will manifest itself as jitter in the timesuccessive scans cross a geometric reference point.

The edge detector 58 of the preferred embodiment utilizes a splitphotocell similar in construction to detector 38 except for orientationof the division perpendicular to the scan path. Each half of the splitphotocell forming detector 58 is essentially identical in area,location, material and temperature. Thus, the use of a split photocellhas advantages over a single cell edge detector in that it will berelatively insensitive to laser light intensity change, temperaturechanges and ambient light.

A second edge detector 62 of similar construction to the edge detector58 is located at a trailing edge of the exposure slot 60. The edgedetector 62 will indicate when spot 34 has passed a fixed terminal pointbeyond the scan path. The time differential as detected between thefirst edge detector 58 and the second detector 62 can be interpretedthrough logic circuitry to indicate the flight time for spot 34 to covera fixed length scan path. Thus, the speed of the spot can be computed.Variations in speed for different scan lines can be detected, and afeedback loop can then be utilized for speed control of the motor drive30.

With regard to the aforementioned, it has been found that as a beam 12is deflected or detuned from the Bragg angle, the efficiency willchange. The scanning system of this invention, however, can beimplemented by introducing an intensity modulator 64 for applying anamplitude modulated correction signal for maintaining laser illuminationat a constant level. The intensity modulator 64 could also be used forcontrol of spot size by either varying the intensity. The use ofdifferent spot sizes can be effectively employed as letters or numbersare created so as to avoid roughened edges and improve characterformation. The system of this invention can also employ two powersources using parallel laser beams with each of the beams being of adifferent diameter and corresponding spot size. This will provide amatrix of dots having different sizes for forming a single generatedcharacter. The different size dot will intermesh to create letters andnumerals having a smoother appearance.

Having thus described the invention, it will be seen that there isprovided a laser scanning system which achieves the various objects ofthis invention and which would be well suited to meet conditions ofpractical use.

As various changes may be made in this system as above set forth, it isto be understood that all matter herein described or shown in theaccompanying drawings is to be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. In a light scanning apparatus comprising meansfor generating a laser beam, means for modulating the amplitude of thebeam, a multifaceted reflective polygon positioned in the beam path,means for rotating the polygon, the beam being reflected from successivefacets of the polygon and sweeping along a scan path to providesuccessive raster lines, a photoreceptor positioned to have the rasterlines extending thereacross, the improvement comprising a beamdeflection system, the deflection system including electro-opticalsensing means, the sensing means being a pair of juxtaposedphotodetectors being juxtaposed along an axis in registry with the scanpath ahead of the photoreceptor, the electro-optical sensing meansdetecting the presence of the beam and in response thereto providing atleast one signal indicative of the presence of the beam with respect tothe longitudinal scan path axis, the signal indicative of the beamcomprising a signal generated from at least one of the photodetectors,means receiving the signal indicative of the presence of the beam and inresponse thereto deflecting the beam toward the scan path axis.
 2. Alight scanning apparatus constructed in accordance with claim 1 whereinthe means modulating the beam includes an acousto-optical deflector. 3.A beam correction system constructed in accordance with claim 1 whereinthe means deflecting the beam includes means for generating a signalcomprising a series of clock pulses, counter means and gate means, thegate means receiving the signal indicative of the presence of the beamand in response thereto transmitting the clock pulse signal to thecounter means.
 4. A beam deflection system constructed in accordancewith claim 3 wherein the means deflecting the beam includes d comparatormeans, each of the photodetectors being electrically coupled to thecomparator means, the comparator means providing a first signal when thebeam is primarily present in one of the photodetectors and a secondsignal when the beam is primarily present on the other photodetector,the counter means including steering means for controlling the directionof count, the steering means receiving the first signal and the secondsignal, the steering means in response to the presence of the firstsignal incrementing the count of clock pulses and in response to thepresence of the second signal decrementing the count of clock pulses,the counter means providing a digital count signal indicative of therequired beam deflection.
 5. A beam deflection system constructed inaccordance with claim 4 wherein the means deflecting the beam includesan acousto-optical deflector and digital to analog conversion means, thedigital to analog conversion means receiving the count signal and inresponse thereto providing a corresponding analog signal, theacousto-optical deflector receiving said analog signal.
 6. A beamdeflection system constructed in accordance with claim 5 includingsummer means for measuring the amount of light falling on theelectro-optical sensing means, an amplitude control intermediate theacousto-optical deflector and the digital to analog converter, theamplitude control receiving a signal from the summer means to maintainconstant the light output from the acousto-optical deflector.
 7. In alight scanning apparatus comprising means for generating a laser beam,means modulating the amplitude of the beam, a multi-faceted reflectivepolygon positioned in the beam path, means for rotating the polygon, thebeam being reflected from successive facets of the polygon and sweepingalong a scan path to provide successive raster lines, a photoreceptorpositioned to have the raster lines extending thereacross, theimprovement comprising position reference means for synchronizing theamplitude modulation of the beam for each successive raster line sweep,the position reference means comprising electro-optical sensing means,the sensing means being positioned along the scan path ahead of thephotoreceptor, the sensing means comprising a pair of juxtaposedphotodetectors, the photodetectors being juxtaposed along an axisperpendicular to the longitudinal scan path axis, each of thephotodetectors providing a signal indicative of the presence of thebeam, means receiving the signal indicative of the presence of the beamfrom each photodetector, the receiving means comprising the signal ofone of the photodetectors with respect to the signal of the otherphotodetector and providing an indication signal when the beam crossesthe axis of juxtaposition, whereby the position of the beam can beprecisely located prior to traversing the photoreceptor.
 8. A lightscanning apparatus constructed in accordance with claim 7 wherein thesensing means comprises a split photodetector.
 9. In a light scanningapparatus comprising means for generating a laser beam, means modulatingthe amplitude of the beam in accordance with a writing program, amultifaceted relective polygon positioned in the beam path, means forrotating the polygon, the beam being reflected from successive facets ofthe polygon and sweeping along a scan path to provide successive rasterlines, a photoreceptor positioned to have the raster lines extendthereacross, a method of correcting for facet plane to rotation axiserror in a polygon, the method comprising the steps of:(a) detecting thepresence of the beam with respect to the longitudinal scan path axis ata zone ahead of the photoreceptor, (b) providing a signal indicating adirection for corrective displacement of the beam comprising a firstsignal when the beam is positioned in one direction with respect to thelongitudinal scan path axis and a second signal when the beam ispositioned on the opposite side of the longidudinal scan path axis and(c) acousto-optically deflecting the beam in response to the signal. 10.In a method for correcting imperfection in a polygon used for laserscanning data, the steps comprising:rotating a polygon; directing a beamof light upon the polygon to produce a light scan path; placing aphotoreceptor in a portion of the scan path; placing a detector inanother portion of said path; dividing said detector along a lineparallel to the scan path into first half and second half portions;detecting the location of said beam when it falls within the detector;and diverting the direction of the scan beam in a direction toward thesecond half portion, when the scan beam falls in the first half portionand diverting the direction of the scan beam toward the first halfportion when the scan beam falls in the second half portion.
 11. In alight scanning apparatus comprising means for generating a laser beam,means for modulating the amplitude of the beam, a multifacetedreflective polygon positioned in the beam path, means for rotating thepolygon, the beam being reflected from successive facets of the polygonand sweeping along a scan path to provide successive raster lines, aphotoreceptor positioned to have the raster lines extending thereacross,the improvement comprising a beam deflection system for deflecting thebeam toward the scan path, the deflection system including first andsecond electro-optical sensing means, the first and second sensing meansbeing positioned on opposite sides of the scan path ahead of thephotoreceptor, at least one of the electro-optical sensing meansdetecting the presence of the beam and in response thereto providing atleast one signal indicative of the presence of the beam with respect tothe longitudinal scan path axis, means receiving the signal indicativeof the presence of the beam and in response thereto deflecting the beamtoward the scan path axis when the presence of the beam is detected inonly one sensing means.